The detection of biomolecules and certain chemicals is important for a wide variety of applications. Applications for biomolecule sensing include medical diagnostics, food safety, and anti-bioterrorism. Conventional biosensors often include fluorescent reporter labels to generate a detection signal.
Label-free biosensors and chemical sensors can directly measure unmodified samples without the need for reporter molecules. Such label-free sensors generally operate based on a change in refractive index due to affinity binding events of biomolecules or chemicals. For example, biomolecules immobilized on the surface of surface plasmon resonance (SPR), fiber optic, and planar waveguide sensors interact with the evanescent field of either the surface plasmon or waveguide mode and cause a refractive index change near the surface of these sensors. However, these evanescent wave sensors are limited in sensitivity, especially for small molecule (e.g. 200-1,000 Daltons) detection, since the surface area is small and the interaction between biomolecules or chemicals and the electromagnetic field is generally fairly weak.